Fuel cells are energy storing systems in which chemical energy is converted into electrical energy through an electrochemical reaction of fuel with oxygen. Since fuel cells are not based on the Carnot cycle, they are clean energy sources having a higher theoretical power generation efficiency than energy using fossil fuels and do not discharge a significant amount of environmental contaminants. Such fuel cells can be used as power sources for small electric/electronic devices, particularly portable devices, as well as for industrial, domestic, and transportation applications.
Fuel cells can be classified into molten carbonate fuel cells which operate at higher temperatures of approximately 500° C. to 700° C., phosphoric acid electrolyte cells which operate at approximately 200° C., and alkaline electrolyte fuel cells and polymer electrolyte membrane (PEM) fuel cells which operate at room temperature or at approximately 100° C. The working temperature and constituent materials of fuel cells are determined depending on the type of electrolyte used in a cell.
Fuel cells can be classified into an external reformer type where fuel is supplied to the anode after being converted into hydrogen-rich gas by a fuel reformer, and a direct fuel supply type or internal reformer type where fuel in gaseous or liquid state is directly supplied to the anode. A representative example of a direct liquid fuel supply type fuel cell is a direct methanol fuel cell (DMFC). DMFCs generally use an aqueous methanol solution as fuel, and a polymer electrolyte membrane with hydrogen ionic conductivity as an electrolyte. It is known that since DMFCs do not require an external reformer and use fuel that is convenient to handle, they have a high potential for use as portable energy sources.
Electrochemical reactions in a DMFC occur when fuel is oxidized at the anode, and oxygen is reduced into water through a reaction with hydrogen ions at the cathode.
Anode Reaction:CH3OH + H2O → 6 H+ 6 e− + CO2Cathode Reaction:1.5 O2 + 6 H+ 6 e−→ 3 H2OOverall Reaction:CH3OH + 1.5 O2 → 2 H2O + CO2
As seen above, one methanol molecule reacts with one water molecule at the anode to produce one carbon dioxide molecule, six hydrogen ions and six electrons. The produced hydrogen ions migrate to the cathode through a polymer electrolyte membrane where they react with oxygen and electrons, which are supplied via an external circuit to produce water. Summarizing the overall reaction in the DMFC, water and carbon dioxide are produced through the reaction of methanol with oxygen. As a result, a substantial part of the energy equivalent to the heat of combustion of methanol is converted into electrical energy.
The polymer electrolyte membrane having hydrogen ionic conductivity acts as a path for the hydrogen ions generated through the oxidation reaction at the anode to migrate to the cathode, and as a separator between the anode and the cathode. The polymer electrolyte membrane requires sufficiently high ionic conductivity to facilitate rapid migration of a large number of hydrogen ions, electrochemical stability, and mechanical strength suitable for a separator, thermal stability at working temperature, ease of processing into a thin film so that its ressitance to ionic conduction can be lowered, and a non-swelling property when permeated by liquid.
Fluorinated polymer membranes such as Nafion (Dupont), Assiflex (Asahi Chemicals), and Flemion (Asahi Glass) are available as polymer membranes for fuel cells. These fluorinated polymer membranes operate relatively well at low temperature, but lose water contained therein at higher temperatures of at least 130° C., thereby causing destruction of the ion channel structure and affecting ionic conductivity. In the case of DMFC, methanol leakage through the membrane occurs and its practicality is low. Also, since the fluorinated polymer membrane is so expensive, it is difficult to commercialize.
To overcome such problems, research aimed at developing a less expensive polymer membrane than Nafion, such as a trifluorstyrene copolymer disclosed in U.S. Pat. No. 5,422,411, has been conducted. However, the less expensive polymer membrane has poor mechanical properties and film forming ability. Also, systems using sulfonated aromatic polymers such as polyimide or polyether sulfone are highly brittle making it difficult to form the membrane. A sulfonic acid group (—SO3H) introduced to provide ionic conductivity increases the brittleness of the system, and thus a stable membrane cannot be formed. To overcome these disadvantages, the rate of sulfonation of the polymer may be lowered or the thickness of the membrane may be increased. In this case, the ion exchange ability of the membrane remarkably decreases, and consequently, the performance of a fuel cell using the electrolyte membrane is lowered.